The present invention relates to a flow meter for measuring a flow rate of fluid, such as a gas, with ultrasonic waves.
FIG. 20 illustrates a conventional flow meter. A first ultrasonic oscillator 32 and a second ultrasonic oscillator 33 provided across a flow passage 31 in which fluid, such as gas, flows transmit and receive ultrasonic wave. A switch unit 34 switches transmitting and receiving operations of the ultrasonic oscillators. A transmitter 35 drives one of the first and second ultrasonic oscillators 32 and 33 to transmit an ultrasonic wave. An amplifier 34 amplifies an ultrasonic wave received by the other oscillator through the switching unit 34 to a predetermined amplitude. A reference comparator 37 compares a voltage of the signal amplified by the amplifier 36 with the amplitude of a reference voltage. A judging unit 38 outputs a signal D upon detecting the first zero-crossing point Ta of the detection signal in time after the comparator 37 detects that the received signal is larger than the reference voltage, as shown in FIG. 21. A repeating unit 39 counts the number of the signals D received from the judging unit 38 and simultaneously transfers the signals D to a controller 42. A time counter 40 measuring a duration of time before the repeating unit 39 counts the signals up to a predetermined number. A flow-rate calculator 41 calculates a flow-rate of the fluid from an output signal of the time counter 40. The controller 42 controls the transmitter 35 according to signals from the flow-rate calculator 41 and the repeating unit 39.
An operation of the conventional flow meter will be described in more detail. First, the controller 42 drives the transmitter 35 and the switching unit 34 to start transmitting an ultrasonic wave from the first ultrasonic wave oscillator 32. The ultrasonic wave is then propagated through fluid and is received by the second ultrasonic wave oscillator 33, and is then amplified by the amplifier 36. The reference comparator 37 outputs a signal C falling when the output signal of the amplifier 36 become larger than the reference voltage. The judging unit 38 outputs a signal D falling at the first zero-crossing point Ta in time after the output signal of the amplifier 36 becomes larger than the reference voltage. The repeating unit 39 transfers the signal D from the judging unit 38 to the controller 42. This operation is repeated N times predetermined, and the duration of the repeating is measured by the time counter 40. Then, the switching unit 34 switches the transmission of the ultrasonic signal from the first ultrasonic wave oscillator 32 to the second ultrasonic wave oscillator 33, and then, the same procedures are executed. The flow-rate calculator 41 receives, from the time counter 40, the duration in a forward direction from an upstream to a downstream of the fluid and the duration in a reverse direction from the downstream to the upstream of the fluid, and calculates a flow rate Q by:
Q=Kxc2x7Sxc2x7v=Kxc2x7Sxc2x7L/2xc2x7(n/t1xe2x88x92n/t2)xc2x7cos xcfx86xe2x80x83xe2x80x83(Equation 1)
where L is an effective distance in a flowing direction of the fluid between the ultrasonic wave oscillators 32 and 33, t1 is the duration of the signal D transmitted N times in the forward direction, t2 is the duration of the signal D transmitted N times in the reverse direction, v is a measured velocity of the fluid, S is a cross section of the flow passage, xcfx86 is an angle formed by a line extending between the ultrasonic wave oscillators 32, 33and the flowing direction, and K is a constant determined according to the flow rate.
The amplifier 36 has its gain adjusted such that the signal received by the ultrasonic wave oscillator is output at a constant amplitude, and peak values range in a predetermined range. More specifically, while the repeating unit 39 counts the number of the signals D up to the predetermined number, the number of times that the peak of the received signal is smaller than the lower limit of a predetermined-voltage range, as shown by dotted line AL in FIG. 22 is counted, and the number of times that the peak of the received signal is larger than the upper limit of the range, as shown by dotted line AH in FIG. 22, is counted. According to the counted numbers, the gain for the subsequence measuring of flow rate is determined. For example, if the number of times that the peak of the received signal is smaller than the lower limit is greater than the number of times that the peak of the received signal is larger than the upper limit, the gain is increased so that the peaks of the received signal denoted by real line A in FIG. 22 stay between the upper limit and the lower limit of the range.
The reference voltage of the reference comparator 37 to be compared with the voltage of the signal from the amplifier 36 is determined for the judging unit 38 to detect the position of zero crossing. More particularly, as shown in FIG. 21, the reference voltage is determined to be a substantial middle between the second peak and the third peak of the received signal in time, thus enabling the judging unit 38 to detect the first zero-crossing point Ta in time after the third peak P3. This assures a margin against the case that the second peak P2 increases or the third peak P3 declines according to a change in the flow rate or a temperature, thus allowing the judging unit 38 to detect the zero-crossing point Ta.
In the conventional flow meter having the above arrangement, the reference voltage to be compared with the voltage of the received signal amplified to a desired voltage is determined by resistance-voltage division implemented by a fixed resistor and a variable resistor. The division however requires the variable resistor to be manually adjusted for determining the reference voltage while monitoring the voltage, thus taking a considerable length of time and possibly creating an error in the adjusting. Further, the reference voltage remains unchanged during the measurement of the flow rate, and the gain of the amplifier for amplifying the received signal is fixed. Therefore, the voltage of the received ultrasonic wave varies during the measurement, and the relationship between the voltage and the reference voltage may accordingly change. For example, when the received signal significantly declines in its amplitude, the third peak P3 used for detecting the arrival of the wave may not reach the reference voltage. Accordingly, the reference comparator may output the signal C at the timing of the fourth peak P4, thus causing the judging unit to detect the first zero-crossing point after the fourth peak P4. As the result, the time counter measures the duration incorrectly, thus permitting the flow-rate calculator to calculate a incorrect flow rate.
A flow meter measures a flow rate of fluid flowing through a flow passage. The flow meter includes first and second oscillators provided across the flow passage for transmitting and receiving an ultrasonic wave, a transmitter for driving the first and second oscillators, a switching unit for switching transmission of the ultrasonic wave between the first and second oscillators, an amplifier for amplifying a signal received from the first and second oscillators, a flow-rate calculator for calculating the flow rate based on a duration of propagation of the ultrasonic wave between the first and second oscillators, a reference comparator for comparing a voltage of the amplified signal with a reference voltage to output a signal which indicates a time point where a relationship between the voltage of the amplified signal and the reference voltage changes, a judging unit for detecting an arrival time of the ultrasonic wave at the first and second oscillators based on the signal output from the reference comparator and the signal output from the amplifier to output a signal which indicates the arrival time, and a reference setting unit. The reference setting unit includes a propagation measuring unit for measuring a duration of propagation of the ultrasonic wave along the flow passage from a start of transmission of the ultrasonic wave to the arrival time detected by the judging unit, and a voltage setting unit for determining the reference voltage based on the duration of propagation and for the reference voltage to the reference comparator.
In the flow meter, the reference voltage is automatically set to a level assuring a margin against a change in the amplitude of the received signal, and the reference voltage is determined readily and accurately according to a change in the amplitude of the received signal during the measurement of the flow rate. This allows the judging unit to precisely detect a desired point or the first zero-crossing point in time after a desired point, e.g. the third peak P3 of the received signal, thus enabling the flow meter to measure the flow rate accurately against a change in the amplitude of the received signal.